The researchers drew inspiration from wind instruments in devising these mechanisms, which they call Acoustruments. The idea is to use pluggable plastic tubes and other structures to connect the smartphone's speaker with its microphone.

The device can then be controlled by acoustically altering sounds as they pass through this system.

"We're providing people with tangible interactivity at basically no cost," said lead author Gierad Laput from Carnegie Mellon's Human-Computer Interaction Institute (HCII).

People who want to control a smartphone application while their eyes need to be focused elsewhere, for instance, might be less distracted by a tangible knob or button made possible with Acoustruments than with trying to glance at a touchscreen.

"Using smartphones as computers to control toys, appliances and robots already is a growing trend, particularly in the maker community.

"Acoustruments can make the interactivity of these new 'pluggable' applications even richer," Laput added.

Applications that use smartphones as virtual reality displays make it impossible to use the touchscreen controls at all. Acoustruments enable users to make adjustments without disrupting the virtual reality experience.

The CMU and Disney researchers have used Acoustruments to build an interactive doll which responds when its tummy is poked, a smartphone case that can sense when it has been placed on a table or is being hand carried, and an alarm clock that provides physical on/off and snooze buttons.

Acoustruments can be made with 3-D printers, with injection moulds, or even by hand in some cases, Laput said.